Making the View Interactive in Android

Drawing a UI is only one part of creating a custom view. You also need to make your view respond to user input in a way that closely resembles the real-world action you’re mimicking. Objects should always act in the same way that real objects do. For example, images should not immediately pop out of existence and reappear somewhere else, because objects in the real world don’t do that. Instead, images should move from one place to another.

Users also sense subtle behavior or feel in an interface, and react best to subtleties that mimic the real world. For example, when users fling a UI object, they should sense friction at the beginning that delays the motion, and then at the end sense momentum that carries the motion beyond the fling.

This lesson demonstrates how to use features of the Android framework to add these real-world behaviors to your custom view.

Handle Input Gestures

Like many other UI frameworks, Android supports an input event model. User actions are turned into events that trigger callbacks, and you can override the callbacks to customize how your application responds to the user. The most common input event in the Android system is touch, which triggersonTouchEvent(android.view.MotionEvent). Override this method to handle the event:

Touch events by themselves are not particularly useful. Modern touch UIs define interactions in terms of gestures such as tapping, pulling, pushing, flinging, and zooming. To convert raw touch events into gestures, Android provides GestureDetector.

Construct a GestureDetector by passing in an instance of a class that implements GestureDetector.OnGestureListener. If you only want to process a few gestures, you can extend GestureDetector.SimpleOnGestureListener instead of implementing the GestureDetector.OnGestureListenerinterface. For instance, this code creates a class that extends GestureDetector.SimpleOnGestureListener and overrides onDown(MotionEvent).

Whether or not you use GestureDetector.SimpleOnGestureListener, you must always implement an onDown() method that returns true. This step is necessary because all gestures begin with an onDown() message. If you return false from onDown(), as GestureDetector.SimpleOnGestureListenerdoes, the system assumes that you want to ignore the rest of the gesture, and the other methods of GestureDetector.OnGestureListener never get called. The only time you should return false from onDown() is if you truly want to ignore an entire gesture. Once you’ve implementedGestureDetector.OnGestureListener and created an instance of GestureDetector, you can use your GestureDetector to interpret the touch events you receive in onTouchEvent().

When you pass onTouchEvent() a touch event that it doesn’t recognize as part of a gesture, it returns false. You can then run your own custom gesture-detection code.

Create Physically Plausible Motion

Gestures are a powerful way to control touchscreen devices, but they can be counterintuitive and difficult to remember unless they produce physically plausible results. A good example of this is the fling gesture, where the user quickly moves a finger across the screen and then lifts it. This gesture makes sense if the UI responds by moving quickly in the direction of the fling, then slowing down, as if the user had pushed on a flywheel and set it spinning.

However, simulating the feel of a flywheel isn’t trivial. A lot of physics and math are required to get a flywheel model working correctly. Fortunately, Android provides helper classes to simulate this and other behaviors. The Scroller class is the basis for handling flywheel-style fling gestures.

To start a fling, call fling() with the starting velocity and the minimum and maximum x and y values of the fling. For the velocity value, you can use the value computed for you by GestureDetector.

Note: Although the velocity calculated by GestureDetector is physically accurate, many developers feel that using this value makes the fling animation too fast. It’s common to divide the x and y velocity by a factor of 4 to 8.

The call to fling() sets up the physics model for the fling gesture. Afterwards, you need to update the Scroller by callingScroller.computeScrollOffset() at regular intervals. computeScrollOffset() updates the Scroller object’s internal state by reading the current time and using the physics model to calculate the x and y position at that time. Call getCurrX() and getCurrY() to retrieve these values.

Most views pass the Scroller object’s x and y position directly to scrollTo(). The PieChart example is a little different: it uses the current scroll y position to set the rotational angle of the chart.

The Scroller class computes scroll positions for you, but it does not automatically apply those positions to your view. It’s your responsibility to make sure you get and apply new coordinates often enough to make the scrolling animation look smooth. There are two ways to do this:

Call postInvalidate() after calling fling(), in order to force a redraw. This technique requires that you compute scroll offsets in onDraw() and callpostInvalidate() every time the scroll offset changes.

Set up a ValueAnimator to animate for the duration of the fling, and add a listener to process animation updates by calling addUpdateListener().

The PieChart example uses the second approach. This technique is slightly more complex to set up, but it works more closely with the animation system and doesn’t require potentially unnecessary view invalidation. The drawback is that ValueAnimator is not available prior to API level 11, so this technique cannot be used on devices running Android versions lower than 3.0.

Note: You can use ValueAnimator in applications that target lower API levels. You just need to make sure to check the current API level at runtime, and omit the calls to the view animation system if the current level is less than 11.

Make Your Transitions Smooth

Users expect a modern UI to transition smoothly between states. UI elements fade in and out instead of appearing and disappearing. Motions begin and end smoothly instead of starting and stopping abruptly. The Android property animation framework, introduced in Android 3.0, makes smooth transitions easy.

To use the animation system, whenever a property changes that will affect your view’s appearance, do not change the property directly. Instead, useValueAnimator to make the change. In the following example, modifying the currently selected pie slice in PieChart causes the entire chart to rotate so that the selection pointer is centered in the selected slice. ValueAnimator changes the rotation over a period of several hundred milliseconds, rather than immediately setting the new rotation value.

If the value you want to change is one of the base View properties, doing the animation is even easier, because Views have a built-inViewPropertyAnimator that is optimized for simultaneous animation of multiple properties. For example: